1. Field of the Invention
The present invention relates to an apparatus for testing the soft handoff functionality of a mobile station, such as cellular telephone acting as the mobile station being handed off between two base stations. More particularly, the present invention relates to an apparatus for testing soft handoff functionality of a Code Division Multi-Access (CDMA) communication system.
2. Description of the Related Art
An explanation of soft handoff and a description of a conventional apparatus to test soft handoff are provided below.
Soft handoff is a handoff between CDMA channels that have identical frequency assignments. Soft handoff provides diversity of Forward Traffic Channel and Reverse Traffic Channel paths on the boundaries between base stations. A traffic channel is defined as a communication path between a mobile station and a base station used by both a user and signaling traffic, the signaling traffic controlling communication between the mobile station and base station. The term xe2x80x9ctrafficxe2x80x9d implies a forward traffic channel (base station to a mobile station) and a reverse traffic channel (mobile station to base station) functioning as a pair.
Before and during soft handoff, a base station and its neighboring base stations send pilot channel signals on a carrier and a MS (Mobile Station) monitors the signal strength of the pilot channels. The pilot channel is an unmodulated direct-sequence spread spectrum signal transmitted continuously to each CDMA base station. A pilot channel allows a MS to acquire the timing of a forward CDMA channel, provide a phase reference for coherent demodulation, and provide a means for signal strength comparisons between base stations for determining when to handoff. The MS measures the pilot strengths and provides a signal back to the base stations reporting the pilot signal strengths.
FIG. 1A illustrates soft handoff between two base stations. FIG. 1A shows a MS moving away from a Base Station (BS1) and approaching a Base Station 2 (BS2), while the MS is receiving a traffic channel from BS1 on the boundary between BS1 and BS2. When the MS detects a pilot signal from BS2 of sufficient strength, the MS reports the pilot strength to BS1. Then BS1 sends the MS a message telling the MS to receive the traffic channel of BS1 and the traffic channel of BS2. The MS then begins to receive both BS1 and BS2 traffic channels simultaneously (providing a diversity of forward link). At this point, both BS1 and BS2 are receiving traffic channels from the MS, which allows the transmission of higher quality traffic data to the network (providing diversity of reverse link). As the MS moves farther away from BS1, the MS will detect a weak pilot strength and report the weak pilot strength from BS1. The MS then begins to receive only the traffic channel from BS2. During the entire process, the MS can make handoffs between BS1 and BS2 without interrupting communication.
FIG. 1B illustrates sector-to-sector handoff for a single base station 100 transmitting over different antennas to separate sectors using different sector units 102 and 104 of the single base station 100. Such sector-to-sector handoff is sometimes referred to as softer handoff. FIG. 1B shows the two sectors 102 and 104 located in the single base station 100 for covering adjacent sectors. Softer handoff occurs when the MS is moving on the boundary between a sector 1 controlled by a sector unit 102 and a sector 2 controlled by sector unit 104.
Soft handoff is a procedure enabling an interruption-free handoff. Manufacturers of CDMA phones assure communication quality by verifying the soft handoff functionality. The following are requirement specifications for a conventional test apparatus for testing soft handoff functionality of a MS:
Be able to simulate two base stations.
Be able to add Additive White Gaussian Noise (AWGN) to the forward link signal.
Be able to control the level of the forward link signal (including the forward link signal of BS1, the forward link signal of BS2 and (AWGN) correctly and with sufficient resolution.
Be able to exchange messages needed during handoff with the MS.
Be able to display the contents of the pilot strength measurement report sent from the MS.
Be able to define the contents of the system parameter message from the base station (some parameters are used to inform the MS of a condition to monitor pilot strength).
FIG. 2 shows one example of a conventional test system for testing soft handoff. The test system includes two base station simulators (BS1) 202 and (BS2) 204, an AWGN generator 206, a test system controller 200, couplers 207, 208, a duplexer 210 and a power divider 212. Generally the test system shown in FIG. 2 and a (MS) 214 are connected using RF cables. An antenna or aerial interface may be used between the duplexer 210 and MS 214 instead of the RF cables depending on the condition of the test.
1. Base Station Simulators 202, 204
Two base station simulators 202 and 204 are needed to simulate two base stations. The MS 214 will make a handoff from one base station simulator to another during testing. FIG. 3 shows a block diagram of a conventional base station simulator as described in more detail below.
2. AWGN Generator 206
The AWGN generator 206 is used to simulate noise and signals sent from other base stations in the field. Some base station simulators have a built-in AWGN generator. The base station simulators in a conventional test system can have built in AWGN generators, so a separate AWGN generator as shown in FIG. 2 is not needed.
3. Test System Controller 200
In order to execute soft handoff between one base station simulator to the other, the two base stations 202 and 204 need to synchronize with each other. The test system controller 200 is used to control both base station simulators, allowing them to handle call processing or message transmission/receptions to and from the MS 214 being tested. The test system controller 200 also controls the AWGN. The test system controller 200 controls the forward link signal for both of the base station simulators 202 and 204 and the AWGN 206. The test system controller 200 further provides for displaying of the contents of the pilot strength measurement report message sent from the MS 214.
4. Couplers 207, 208 and Duplexer 214
The couplers 207 and 208 are used to combine the forward link signals of the base station simulators, and the AWGN and provides those signals to a duplexer 210. The duplexer directs the signal from the coupler 208 through either a cable or antenna to a mobile station 214.
5. Power Divider 212
The reverse link signal from the MS 214 is directed by the duplexer which then feeds the signal received to both base station simulators using the power divider 212.
6. Conventional Base Station Simulator Circuitry (FIG. 3)
FIG. 3 shows the components of a conventional base station simulator, such as either 202 or 204. The conventional components of the base station simulator includes a transmit (TX) baseband processor 302 which generates a digital forward link signal with a desired indication for amplitude. The digital forward link signal is converted to an analog signal in a digital to analog (D/A) converter 304. The output of the D/A converter 304 is then provided through a quadurature modulator 306 and upconverter 308 for transmission from the base station simulator. A signal received by the base station simulator shown in FIG. 3 is downconverted in downconverter 310, demodulated in quadrature demodulator 312 and converted from an analog to a digital signal in the A/D converter 314. The output of the A/D converter is provided to a receive (RX) baseband processor 316 which functions to further demodulate the received IF digital, decode the demodulated data and provide the demodulated data to the control part 320. The output of the A/D converter further provides its output to a MS performance measurement unit 318 which measures the accuracy of the signal level and determines modulation accuracy of a MS. The control part 320 controls the call processing status and hardware settings that simulate a base station according to the condition that a test operator sets via the user interface 322. The user interface 322 can include a display screen and keypad, or a remote controlling computer.
The conventional system for testing the soft handoff function of a MS has the following drawbacks:
The test system needs two base station simulators and an external controller (including hardware and software). The cost of a conventional test system is, thus, more than double the cost of one base station simulator.
The individual forward link signal levels of BS1202, BS2204 and AWGN 206 are specified relative to the total forward link signal power, i.e. BS1+BS2+AWGN. Maintaining accurate forward link signal levels is then critical to testing the soft handoff functionality of the MS 214, and maintaining the accuracy is very difficult since it typically requires rigorous calibration for each signal source.
The test system operator needs to operate the two base station simulators 202 and 204 and a test system controller 200. The different connections for these components during a test procedure, and steps for operating each of the components during the test means an operator must perform a number of complicated tasks. A test system controller might be configured which could automatically make the connections and automatically preform the steps required for testing to minimize the operator tasks, but the cost for development of such a system could be high.
In accordance with the present invention, a system is provided for testing soft handoff functionality which avoids the cost and space required for the redundant components of two separate base station simulators. The system in accordance with the present invention further provides improved accuracy in generating signal strength levels for the forward link signals. The system further provides a reduction in complexity for the test procedure relative to a conventional system for testing soft handoff functionality. The system further provides testing for soft handoff between two base stations as illustrated in FIG. 1A, or softer handoff between two sectors of a single base station as shown in FIG. 1B.
Referring to FIG. 4, the base station simulator 400 in accordance with the present invention includes two digital baseband processors 402 and 404 for generating forward transmission signals to simulate the signals from the two base stations (BS1) and (BS2). The two digital base station signals have gain digitally controlled using multipliers 407-409 and the signals are added together digitally using a digital adder 410 before conversion to an analog signal using an A/D converter 412. Adding of the digital signals before converting to analog in A/D converter 412 increases measurement accuracy, as opposed to coupling the base station analog signals together. Further, adding of a digital noise signal from an AWGN generator 406 before conversion to analog can further improve accuracy.
The system further includes components for upconverting the signal from the D/A converter 412 and transmitting the upconverted signal to a mobile station 430. Additionally, the system includes components for downconverting the signal transmitted from the mobile station 430 and measuring the performance of the MS 430, similar to the components of a single base station simulator shown in FIG. 3. But, by combining the signals from two baseband processors 402 and 404 before upconversion using a single upconversion system, and by using a single system for downconversion, the additional cost and space required for the redundant parts from two separate base station simulators is avoided. Further, control of the combined baseband processors 402 and 404, as opposed to separate controls for separate base station simulators, simplifies operation of the system in accordance with the present invention.